8. Abstract We will investigate Herpes simplex virus type 1 (HSV-1) infection of neural progenitor cells (NPCs) in our R01 application. In immunocompetent individuals, HSV-1 infection is the most common cause of encephalitis (HSE). Although antiviral therapy has improved its prognosis, the majority of HSV-1 encephalitis survivors suffer from permanent neurological sequelae. The chronic lesions in HSE patients have been observed in regions associated with memory formation, which include the hippocampus and associated limbic structures. The severity of sequela in HSE is related to the severity of damage to these regions. HSV-1 displays tropism for hippocampus (subgranular zone, SGZ) and the subventricular zone (SVZ) of the lateral ventricles. Both regions are rich in NPCs, which differentiate into neurons and glial cells. These regions are thus major niche areas for adult neurogenesis. Despite the pivotal role played by the NPCs in adult neurogenesis, the effect of HSV-1 on NPCs proliferation, differentiation and migration is largely unknown. Furthermore, whether NPCs may represent reservoirs of HSV-1 in the CNS is unknown. Here, we aim to model the interaction of HSV-1 with NPCs using three-dimensional (3D) cultures of human CNS cells derived from induced pluripotent stem cells (hiPSCs); we will also build on a mouse model of encephalitis to enable experiments not feasible in human cells in vitro. In Aim 1, we will investigate early-stage effects of HSV-1 infection on proliferation, differentiation, and migration of NPCs derived from hiPSCs in three-dimensional (3D) culture systems that we have designed; the model incorporates NPCs and their derivatives. In Aim 2, we will employ a mouse model of encephalitis to investigate the distribution of HSV-1 in the hippocampus and SVZ, the effects of HSV-1 infection on NPCs proliferation and analyze NPCs in surviving mice for periods ranging from 6 months to 1 year. Also, we will determine if there is evidence of HSV-1 latency and reactivation in NPCs. In Aim 3, we will investigate: i) HSV-1 latency in NPCs; ii) the effect of HSV-1 infection on pathways regulating the differentiation fate of NPCs pathways regulating; and iii) the consequences of persistent infection on NPCs function. The rodent and the iPSC data will be synthesized. We hypothesize that i) HSV-1 impairs NPC processes required for neurogenesis; ii) HSV-1 can establish latency in NPCs. What is learned from this project will improve our understanding of HSV-1 encephalitis. We possess the relevant expertise for the proposed work.